This invention relates to a method and apparatus for generating a train of pulses variable in N selectable increments during a period T. Such pulse trains can be used to generate a dc voltage if they are averaged in time by means of a filtering element. If, of the maximum possible number N of increments during the period T, n (n = 0, 1,2 . . . N) increments are generated, and the number n is varied, a continuously variable dc voltage can be taken from the output of the filtering element.
As is generally known, dc voltages derived from pulse voltages by means of filtering elements have a hum voltage superimposed thereon whose amplitude is dependent on the design of the filtering element. If an extremely low hum voltage is required in certain applications, a considerable amount of filtering circuitry is needed.
If, for example, the pulse train variable in N increments were generated during the period T by successively increasing the duration of a rectangular pulse in n increments, i.e. by generating a rectangular pulse with a mark-to-space ratio variable as a function of n, a very high hum voltage would be obtained at a unity mark-to-space ratio; this hum voltage would necessitate expensive filtering elements.
If, on the other hand, the pulse train were generated as invividual pulses distributed over the period T, the pulse train obtained would have a very low hum voltage after filtering, but the large number of leading and trailing pulse edges would be disadvantageous. These pulse edges cause the components generating them, particularly semiconductor devices, to be turned on and off, so the dissipation of these components is variable as a function of the number n. This temperature dependence also appears in the filtered dc voltage.